The invention described herein may be manufactured and used by or for the Government of the United States of America for governmental purposes without the payment of any royalties thereon or therefor.
(1) Field of the Invention
The present invention is directed to a sonar transducer with a tuning plate and a tuning fluid having increased maximum power which may be radiated from a given transducer by increasing the radiation resistance seen by the transducer. The present invention also is directed to providing a method for setting the mechanical resonant frequency of a sonar transducer above or below the value which is characteristic of the transducer without the tuning system.
(2) Description of the Prior Art
In general, the power which may be radiated in a fluid medium by a transducer is proportional to the product of the area of the radiating surface, the square of the surface velocity, and the characteristic impedance (sound speed times fluid density) of the medium. Thus to increase the radiated power, either the surface area, or the velocity, or both, must be increased. However, the radiating surface area of a sonar transducer is usually limited by the requirement to produce a specific directional characteristic (beam pattern). In addition, the maximum velocity which may be achieved is limited by physical constraints on the transduction material, such as maximum electrical field or maximum mechanical strain.
In a sonar transducer in which body stress in the active transduction material (such as, for example, electrostrictive or magnetostrictive materials) is used to convert electrical energy into mechanical energy, the optimum design with respect to power output and bandwidth is achieved when the characteristic mechanical impedance (density times sound speed times cross sectional area) of the transduction material is matched to (i.e. equals) that of the acoustical load. Since the characteristic impedance (density times sound speed) of the transduction material and the fluid medium are inherent material properties, an attempt to achieve the desired match of the characteristic mechanical impedances may take the form of a mechanical area transformation. In the design of a longitudinal vibrator or tonpilz such an impedance match is often approximated by employing a piston whose area is much greater than the area of the active transduction material. In particular, the characteristic impedance of typical piezoelectric ceramic materials (such as lead zirconate titanate) is in the order of 34,000,000 MKS Rayls, whereas the characteristic impedance of sea water is about 1,500,000 MKS Rayls. This would indicate an area ratio of about 23 to 1 to achieve the optimum match of the characteristic mechanical impedance. The dimensions of the radiating piston are generally dictated by directivity considerations, leaving only the area of the ceramic as a design variable. However, use of an area ratio of 23 to 1 would result in a very fragile ceramic configuration. Other considerations, such as withstanding hydrostatic pressure and explosive shock, dictate that the maximum practical area ratio is in the order of 6 to 1. This results in about a 4 to 1 mismatch between the sonar transducer and the fluid medium. For these reasons, it has never been possible to achieve a perfect match.
Accordingly, it is an object of the present invention to provide a method for designing a transducer so that the radiation resistance seen by the transducer is increased to facilitate higher power output.
It is a further object of the present invention to provide a method for designing a sonar transducer having a mechanical resonant frequency which may be decreased or increased.
It is still a further object of the present invention to provide a smaller, lighter sonar transducer.
The foregoing objects are achieved by the method and the sonar transducer of the present invention.
In accordance with the present invention, a method for maximizing the radiated power of a transducer broadly comprises the steps of providing a transducer system comprising a transducer operating at a frequency f and having a face, a tuning fluid medium having a density of xcfx811 and a speed of sound c1, a tuning plate having a density xcfx81p and a thickness t spaced a distance s from the transducer face, and an external fluid medium having a density xcfx812 and a speed of sound c2; and tuning the transducer by choosing xcfx81p, t, and s so that                               tan          ⁡                      (                          2              ⁢                              xe2x80x83                            ⁢              π              ⁢                              xe2x80x83                            ⁢              f              ⁢                              s                                  c                  1                                                      )                          =                                            ρ              1                        ⁢                          c              1                                            2            ⁢                          xe2x80x83                        ⁢            π            ⁢                          xe2x80x83                        ⁢            f            ⁢                          xe2x80x83                        ⁢                          ρ              p                        ⁢            t                                              (        1        )            
Further, in accordance with the present invention, a method for changing the resonance frequency of a transducer is provided. The method comprises the steps of providing a transducer system having a transducer with a face and an operating frequency f, a tuning plate spaced from the transducer face by a distance s, and a fluid medium between the transducer face and the tuning plate having a density xcfx811 and a speed of sound c1; and decreasing the resonance frequency if the expression below is positive or increasing the resonance frequency if it is negative.                               ρ          1                ⁢                  c          1                ⁢                  cot          ⁡                      (                          2              ⁢                              xe2x80x83                            ⁢              π              ⁢                              xe2x80x83                            ⁢              f              ⁢                              s                                  c                  1                                                      )                                              (        2        )            
The present invention also relates to a transducer system for use on a vehicle traveling through an external fluid medium having a density xcfx812 and a speed of sound c2. The transducer system broadly comprises a transducer having a radiating face and an operating frequency f, rigid wall means for positioning the transducer relative to an exterior wall of the vehicle, a tuning plate for separating the transducer from the external medium, which tuning plate has a density xcfx81p and a thickness t and being spaced from the radiating face by a distance s, and a tuning fluid positioned intermediate the tuning plate and the radiating face, which tuning fluid has a density xcfx811 and a speed of sound c1. The system has a complex specific acoustical impedance at the radiating face in accordance with the equation:                               Z          ⁡                      (                          s              ,              f                        )                          =                                            [                                                                                          (                                              2                        ⁢                                                  xe2x80x83                                                ⁢                        π                        ⁢                                                  xe2x80x83                                                ⁢                        f                        ⁢                                                  xe2x80x83                                                ⁢                                                  ρ                          p                                                ⁢                        t                                            )                                        2                                                                              ρ                      2                                        ⁢                                          c                      2                                        ⁢                                          ρ                      1                                        ⁢                                          c                      1                                                                      +                                                                            ρ                      1                                        ⁢                                          c                      1                                                                                                  ρ                      2                                        ⁢                                          c                      2                                                                                  ]                        ⁢                          ρ              1                        ⁢                          c              1                                +                      i            ⁢                          xe2x80x83                        ⁢                          ρ              1                        ⁢                          c              1                        ⁢                          cot              ⁡                              (                                  2                  ⁢                                      xe2x80x83                                    ⁢                  π                  ⁢                                      xe2x80x83                                    ⁢                  f                  ⁢                                      s                                          c                      1                                                                      )                                                                        (        3        )            